1. Field of the Invention
This invention relates to a brick casting method of stave coolers used for cooling the furnace walls of blast furnaces, or the like. The stave coolers are generally arranged between the outer shell and the inner refractory brick wall of a blast furnace, for example.
2. Description of the Related Art
Cracks often occur in bricks when the bricks are cast in the stave cooler in accordance with the prior art technique. To eliminate the problem of cracks it has been a common practice to bond a heat-insulating buffer material to both side surfaces of the bricks for casing the bricks in the stave coolers, as disclosed, for example, in Japanese Patent Publication Nos. 8241/1977 and 31811/1977.
The method described above has been employed on the basis of discovery that the cracks develop on both inner and outer surfaces of the bricks when they are cast in a naked state with molten iron of near 1,300.degree. C. and their stability required as the brick for a hearth stave is thereby lost. The function of this heat-insulating buffer material is to mitigate the thermal impact caused on the bricks and to avoid the cracking of the bricks due to the shrinkage of the stave cooler itself. For this purposes a ceramic felt or the like has been conventionally used as the heat-insulating buffer material.
Further, it is known to apply asbestos on the lower side of the bricks embedded in stave coolers as disclosed in No. 3, March 1967.
Meanwhile it has been found by the present inventors that the above-mentioned prior arts have the following problems.
In the initial casting step where the bricks are arranged in a casting mold and molten pig iron cast around the bricks, every surface of the bricks arranged in the mold is exposed instantaneously to high temperatures around 1,200.degree. C. and severe thermal impact, causing inevitably cracks in the bricks.
According to the above-mentioned prior arts, the heat-insulating buffer material is applied on both upper and lower surfaces of the bricks so that the thermal impact on both surfaces may be mitigated, but the thermal impact on the back side surface cannot be prevented and causes cracking of the brick therefrom.
If stave coolers embedded with bricks suffering the internal cracks caused as above are used in furnace walls of a blast furnace, the internal cracks propagate and expand due to thermal expansion of the bricks caused by the furnace heat after the consumption of the innermost furnace wall bricks, and in combination of severe friction action on the bricks caused by the furnace charges falling down along the furnace walls, the bricks will be wore off only in several years of service, thus lowering the heat insulating ability of the embedded bricks, hence promoting the wearing and consumption of the stave coolers, resulting in a shortened service life of the stave coolers.
Generally, the state coolers are fitted to the hearth walls and used in such a manner that the embedded refractory bricks are arranged in horizontal rows around the inside of the hearth wall. In accordance with the prior arts (Japanese Patent Publication Nos. 8241/1977 and 31811/1977) described above, therefore, the heat-insulating buffer material is placed on the upper and lower surfaces of the refractory bricks.
The heat-insulating buffer material must maintain its shrinkability even after the casting so that it can offset the thermal expansion of the refractory bricks caused when they are exposed to the heat of the furnace gas.
When the heat-insulating buffer material is used in the manner as described above in the wall of the blast furnace, the following problems are encountered.
Namely, the combination of the weight of the refractory bricks and the load acting downward in a vertical direction on the bricks by charge materials such as iron ore and coke filling the furnace and descending thereinalong with the formation of a hot metal compress and shrink the shrinkable heat-insulating buffer material which is positioned between the lower surface of the bricks and the cast iron of the stave. This moves downward the bricks minutely and a gap develops along the boundaries between the bricks and the cast iron on the upper surface of the bricks. Though the heat-insulating buffer material is also disposed on the upper surface of the bricks and though the buffer material maintains its shrinkability, it cannot expand enough to make up the gap developping on the upper surface of the bricks.
If the furnace is operated with the stave coolers having the gap left on the upper surface of the bricks, the high temperature furnace gas enters this gap. Therefore, the refractory bricks are heated from their two surfaces, that is, from the working surface facing the inside of the furnace and from the upper surface. When the bricks are heated on the two surfaces, corrosion of the bricks on those surfaces proceeds rapidly. Accordingly, the bricks are at first corroded at the corners of the upper surface and the working surface and wear away into a triangular shape and finally, when the apex of the triangle reaches the back of the bricks, fall-off of the bricks occurs inevitably.
As described above, the method of casting the bricks with both of their side surfaces covered with the heat-insulating buffer material involves the problem of durability of the bricks.
On the other hand, regarding the back surface of the refractory bricks, if no cover of buffer material is applied to the back surface of the refractory bricks according to the prior arts, this side receives thermal impact which is substantially same as the thermal impact given to the naked brick when they are cast with the molten iron of near 1,300.degree. C. Accordingly, it has been often difficult to prevent the cracking of the refractory bricks at the time of casting.
In other words, the cracks of the bricks occur frequently on the back side at the time of casting and they reduce drastically the ability and function of the stave cooler to retain the embedded bricks therein.
In this aspect, too, the prior arts are not free from the problem of low durability of bricks.